High-power cathode

ABSTRACT

A filament arrangement in a bolt-cathode electron source that heats the bolt-cathode by electron bombardment causing the boltcathode to emit a beam of electrons. The filaments are positioned adjacent to the bolt-cathode and at a distance from the boltcathode sufficient to render the geometry of the bolt-cathode independent from the geometry of the filaments and the current flow in the filaments insensitive to the temperature of the filament.

'--" vwawva l. ll/Cl! U1] [72] Inventor Joel H. Fink [56] References Cited Plmbulflht UNITED STATES PATENTS g g g' 2 3,462,635 8/1969 Broers 313/311 Patented 3 2,117,842 5/1938 George 313/336 [73] Assi nee w c Co 86 2,509,053 5/1950 Calbick 313/337 x g 3,418,526 12/1968 Simon m1 219/121 (EB) Pittsburgh, Pa.

Primary Examiner-John W. l-luckert Assistant ExaminerWilliam D. Larkin Attorneys-F. H. Henson and C. F. Renz [54] HIGH-POWER CATl-IODE 7 Claims6 Drawing Figs ABSTRACT: A filament arrangement in a bolt-cathode elec- [52] US. Cl. 313/338, tron source that heats the bolt-cathode by electron bombard- 219/ 121 EB, 313/346, 313/357 ment causing the bolt-cathode to emit a beam of electrons.

[5 1] Int. Cl. H01] 1/20 The filaments are positioned adjacent to the bolt-cathode and [50] Field of Search 313/337, at a distance from the bolt-cathode sufficient to render the 341, 344, 270, 347, 310, 305, 7 l 357, 299, geometry of the bolt-cathode independent from the geometry 335-336, 338, 302, 304, 103; 2l-9/l2l EB; l3/3l; of the filaments and the current flow in the filaments insensi' 250/495 TC, 49.5 TE, 49.5 Tl, 49.5 Pl tive to the temperature of the filament.

TO VOLTAGE L/ SOURCE PATENTEDrmv 1 e ISTI WITNESSES INVENTOR JqgiH. Fink (0), 2 1 GLIY/Z/LZK ATTORNEY 1 iiiGa-Powsa cx'mona BACKGROUND OF THE INVENTION Field of the Invention This invention is related in.general to electron-emitting sources and in particular to an improvement in bolt-cathode electron-emitting sources.

Description of the Prior Art The bolt-cathode currently in use utilizes a filament in the form of a cylindrical coil with the bolt-cathode inserted in the coil. A number of. design factors limit the spacing of the coil from the cathode which results in several operational disadvantages. A limitation on coil diameter and the necessity of using a large diameter bolt-cathode to generate a maximum level of electron emission results in the operation of the filamentcoil under temperature-limited conditions. This results in unstable operationjnasmuch as the filament coil current is extremely sensitive to temperature change.

Furthennore, the requirement for filament rigidity limits the practical diameter of the filament coil which in turn limits the diameter of the bolt-cathode. This restriction on bolt-cathode diameter limits significantly the beam current generated by the bolt-cathode.

SUMMARY OF THE INVENTION The invention utilizes filament coils which are positioned adjacent to the bolt-cathode. The filament coildesign is not restrictedby the bolt-cathode dimensions. Furthermore, the spacing of the filament .coils from the bolt-cathode is not limited to the extent encountered in the concentric arrangement o'f th 'et ilament coil and the bolt-cathode. The spacing of the filament-coils from the bolt-cathode permits operation of the filament coils under space charge limited conditions which render filament coil current virtually insensitive to filament temperature. This feature provides overall stability not available in the single concentric coil design.

The arbitrary spacing of filament coils from the boltcathode permits selection of a bolt-cathode of sufficient diameter to provide the desired electron beam current.

.BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, there is illustrated in FIG. 1 an electron-emitting source 1, comprised essentially of an elongated solid rod cathode element 3 of a material such as tungsten, tantalum or molybdenum, coil filaments 5 and 7, and heat shield 11. The development of this type of cathode utilizing a tungsten rod is recognized to be the result of the efforts of E. B. Bas of the Swiss Federal Institute of Technology, Zurich, Switzerland. This development of an elongated solid rod cathode element from a high-temperature material exhibiting a low evaporation rate, identified as a bolt-cathode, is discussed in an article by E. B. Bas appearing in Z. Angew Phys. 7, 337(1955).

In FIG. 1 one end of the bolt-cathode 3 is secured in a suitable holder or support 2 while the planar end 13 of boltcathode 3 represents the cathode-emitting surface. The subject of this invention is not in the basic development of the cathode element 3 but rather is concerned with an improvement in the structure of the heater coil arrangement used with the bolt-cathode emitter.

The conventional concentric arrangement of the boltcathode 8 and the heating filament 9 is illustrated in the drawing as prior art. As is well known in the art, filament coil 9 can be heated by the passage of current and alter attaining a critical temperature it is capable of emitting electrons. Boltcathode 8 is heated by coil 9 which not only radiates energy but serves as an auxiliary cathode in a bolt-bombarding circuit. In this structure the electrons emitted are accelerated by a field established between coil 9 and bolt-cathode 8 and constitute a bombarding current which strikes the surface of boltcathode 8. As a result of the initial heating filament coil 9 and the electron bombardment of the bolt-cathode 8, the temperature of bolt-cathode 8 is increased to a level at which electrons are emitted from surface I3. The electrons thus emitted can be accelerated by a suitable accelerating field (not shown) as an electron beam 15 from surface 13 to a target element or anode (not shown).

In analyzing the operation of this single filament coil embodiment of the bolt-cathode several operational disadvantages are apparent. The concentric arrangement of bolt 8 and coil 9 restricts the coil diameter as well as the bolt diameter and thereby limits the spacing between the coil and bolt. The coil diameter is restricted by the requirement for coil rigidity. As determined experimentally and reported by E. B. Bas in Physics 7, 337,(l955), the inner diameter of filament coil 9 should be less that about 6 to 8 times the filament wire diameter in order to provide a reliable coil design. Thus a coil with a large inner diameter must be made from a wire proportionately large diameter. The large-diameter wire limits the pitch of the coil and consequently reduces the practical length of the coil. The apparent advantages of large-diameter wire are further offset by the requirement of increasing heating current as a function of the square of the radius of the wire in order to maintain a desired level of joule heating per unit length. Practical operating current limits do not permit arbitrary selection of wire size. It will be shown further that this. limitation on coil size restricts the scope and stability of the.

operation of the conventional bolt-cathode.

The electron emission density of the bolt-cathode 8 is a function of bolt temperature and bolt diameter. The maximum practical operating temperature is limited by the evaporation rate of tungsten bolt-cathode 8 which in turn establishes the useful operating life of the cathode. Therefore, the maximum electron beam current level available in the concentric fila ment coil-cathode arrangement is dependent on the diameter of the filament coil, the diameter of the bolt-cathode and the operating temperature of the bolt-cathode.

In order to realize maximum electron beam current from cathode-emitting surface 13 in the concentric cathode-filament design, the maximum diameter boltcathode as dictated by the filament coil inner diameter is utilized. The use of a maximum diameter bolt-cathode reduces the spacing between bolt-cathode 3 and filament coil 9 to a minimum. This results in a high-perveance structure wherein perveance is dependent on the geometry and spacing of coil 9 and bolt-cathode 8. A derivation of equations representing current and voltage as a function of spacing for various electrode geometries is presented in Vacuum Tubes, K. R. Spangenberg, Chapter 8. 1948. The high-perveance structure in the conventional boltcathode configuration results in a low bombarding voltage producing a high bombarding current.

In this configuration, in order to achieve reasonable bombarding power and suitable bolt-cathode operating temperatures, the voltage must be increased until the structure is operating in a quasi-space charge limited mode. In this mode of operation the portion of filament coil 9 is operated under temperature-limited conditions whereas the remaining portion of filament coil 9 is operated under space charge limited conditions. This operative mode is comparable to filament coil operation solely under temperature-limited conditions in that the bombarding current of filament coil 9 is extremely sensitive to heater coil temperature changes.

The heating of bolt-cathode 8 by the bombarding electrons and radiant energy emitted by coil 9 establishes a bolt temperature which exceeds the temperature of filament coil 9. As

a result of this temperature difference, coil 9 temperature is increased by radiation energy emitted by bolt-cathode 8. The temperature-sensitive coil 9 responds to this radiant energy by increasing the bombarding current which in turn results in further heating of bolt-cathode 8 and a runaway condition is developed. In an efiort to control this unstable condition in the concentric coil bolt-cathode design external feedback circuits (not shown) are utilized.

. In FIG. I the single coil 9 of the prior art cathode configuration is eliminated and replaced by a pair of filament coils and 7 which are disposed adjacent and substantially parallel to the longitudinal axis of bolt-cathode 3. Coils 5 and 7 are electrically connected through conducting heat shield 11 to a voltage source (not shown). This coil arrangement overcomes the limitation on filament coil diameter, bolt-cathode diameter and consequently the spacing of coil and bolt-cathode experienced in the conventional bolt-cathode configuration. As a result of this arrangement improved operational and physical stability of the structure is provided.

Of primary importance is the unrestricted control of the spacing between the filament coils 5 and 7 and bolt-cathode 3. A low perveance structure is established by the controlled spacing of the filament coils 5 and 7 and the bolt-cathode 3. This spacing results in a relatively low bombarding current being generated for a given bombarding voltage. A low perveance structure permits space charge limited operation with a low-filament coil operating temperature. Therefor, the filament coil current is not temperature limited. The bombarding current is a function of filament coil voltage, and the bombarding power which is a product of the current and voltage, is substantially independent of filament coil temperature.

The use of two filament coils is merely illustrative of a typical design for it is apparent that additional filament coils could be included to increase the bombarding power. Likewise the space relationship of the filament coils to the bolt-cathode is not limited to the parallel arrangement illustrated.

The diameter of coils S and 7 is not dependent on the diameter of bolt-cathode 3 and, therefore, can be wound to inner diameters which will afford desired rigidity. In FIG. 2 a coil design is depicted which illustrates an elliptically wound coil which improves on the cylindrical coil by positioning the remote side of the coil as close to the bolt-cathode 3 as possible so as to achieve an optimum bombarding current.

The stable space charge limited current operating conditions described above can be achieved, for example, by establishing a distance d between bolt-cathode 3 of 0.90-inch diameter and filament coils 5 and 7 of 0.025 inches and applying a filament coil voltage of 300 volts DC which will produce a filament coil current of approximately 400 It is likewise apparent that having removed the physical limitations inherent in the concentric-coil bolt-cathode design that the diameter of bolt-cathode 3 can be increased and thereby increase the cathode emitting surface 13 and the current level of electron beam 15.

The bolt-cathode 3 of FIG. 1 represents a typical pure metal bolt-cathode design. The bolt designs illustrated in FIG. 4 represent equally suitable cathode elements. Inthe boltcathode design of FIG. 4A the cathode is in the form of a disc 30 of material such as tantalum which is fixed onto one end of tungsten rod 32. In FIG. 48 a tantalum cathode 36 is inserted in a recess provided in one end of tungsten rod 38. The bolt operating temperature required for electron emission from a tantalum cathode is lower than the temperature required for tungsten cathode emission.

The heat shield 11 of FIG. 1 can be electrically isolated from filament coils 5 and 7 and the geometry of this shield changed so that it functions as a grid member in either the beam-forming region or in the electron-bombarding region.

In the embodiment of FIG. 3, there is illustrated a typical control grid 20 which can be utilized for both controlling the bombarding current striking the surface of bolt-cathode 3 and focusing the bombarding current onto a particular portion of the bolt-cathode.

While particular embodiments of the invention have been described, it should be understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and arrangement of parts, elements and components can be resorted to without departing from the scope and spirit of the present invention.

What I claim is:

I. An electron-emitting source comprising, an elongated rod cathode member including an electron-emitting element disposed at one end of said elongated rod cathode member for emitting electrons at elevated temperatures, a plurality of discrete electron-emitting means to electron-bombard the surface of said elongated rod cathode member to heat said elongated rod cathode member to said elevated temperatures, and support means operatively connected to the opposite end of said elongated rod cathode member.

2. An electron-emitting source as claimed in claim 1 further including a heat shield, said elongated rod cathode member and said electron-emitting means inserted within said heat shield.

3. An electron-emitting source as claimed in claim 1 wherein each of said electronemitting means is a coil having an effective diameter which is smaller than the effective diameter of said elongated rod cathode element.

4. An electron-emitting source as claimed in claim 1 wherein said electron-emitting element is in the form of a disc having a flat electron-emitting surface.

5. An electron-emitting source as claimed in claim I wherein the material from which said elongated rod cathode member is fabricated is tungsten and the material from which the electron-emitting element is fabricated is tantalum.

6. An electron-emitting source as claimed in claim I wherein each of said electron-emitting means bombard less than the full perimeter of said elongated rod cathode member.

7. An electron-emitting source as claimed in claim 3 wherein said electron-emitting means are elliptical coils. 

1. An electron-emitting source comprising, an elongated rod cathode member including an electron-emitting element disposed at one end of said elongated rod cathode member for emitting electrons at elevated temperatures, a plurality of discrete electron-emitting means to electron-bombard the surface of said elongated rod cathode member to heat said elongated rod cathode member to said elevated temperatures, and support means operatively connected to the opposite end of said elongated rod cathode member.
 2. An electron-emitting source as claimed in claim 1 further including a heat shield, said elongated rod cathode member and said electron-emitting means inserted within said heat shield.
 3. An electron-emitting source as claimed in claim 1 wherein each of said electron-emitting means is a coil having an effective diameter which is smaller than the effective diameter of said elongated rod cathode element.
 4. An electron-emitting source as claimed in claim 1 wherein said electron-emitting element is in the form of a disc having a flat electron-emitting surface.
 5. An electron-emitting source as claimed in claim 1 wherein the material from which said elongated rod cathode member is fabricated is tungsten and the material from which the electron-emitting element is fabricated is tantalum.
 6. An electron-emitting source as claimed in claim 1 wherein each of said electron-emitting means bombard less than the full perimeter of said elongated rod cathode member.
 7. An electron-emitting source as claimed in claim 3 wherein said electron-emitting means are elliptical coils. 